1. Field of the Invention
This invention relates to a focusing screen adapted to be mounted in the optical viewing path of an optical device.
2. Description of the Prior Art
Cameras of the single lens reflex type and also of the range finder type generally include a focus indicating system utilizing an image splitting bi-prism. In this focus indicating systems, a focusing screen having an image splitting bi-prism in the center of it is arranged in a plane in coincidence with a plane in the range finder in which a real image of the object to be photographed is formed. On this focusing screen, each half of the image in the finder is displaced in the defocus (out of focus) condition, and it is only when the image lies in the proper plane of the focusing screen that the two halves of the image do not appear to have any relative displacement. The two halves of the image in the focus condition remain colored in the color inherent to the image.
On the other hand, a cinecamera provided with a focus indicating system of the double image coincidence type is disclosed in U.S. Pat. No. 3,618,498. In this focus indicating system, a dichroic mirror is disposed within the photographic lens system so that double images mutually complementary in color are produced in the defocus condition. However, a focus indicating system of the colored image type using no dichroic mirror and applicable to a single lens reflex camera does not yet exist.
Japanese Pat. Nos. 9474/1955 and 12817/1979 disclose focusing screens of the double image coincidence type utilizing a diffraction lattice. It is described in these Japanese patents that in a view finder having such a focusing screen disposed therein, the double images viewed in the defocus condition are colored by the dispersion of the diffraction lattice. However, the coloring by this dispersion occurs only incidentally in the marginal portion of each image during the separation of the double images and the whole of each image is not colored and therefore, this coloring hardly contributes to the enhancement of the accuracy and ease of the focus detection.